In many electronic circuits, a reference voltage is often used to compare voltages, determine the state of a memory cell, perform analog to digital conversions, and the like. A stable reference voltage is therefore often crucial to the successful operation of many electronic circuits.
Today, with the miniaturization and integration of most electronic circuits onto semiconductor devices ("chips"), reference voltages are commonly provided by circuits which utilize the gap between the valence and conduction bands (as shown in FIG. 1) found in semiconductor substrates (e.g., silicon and gallium arsenide). The gap between these energy bands, E.sub.g is commonly referred to as the band-gap. E.sub.g fluctuates with temperature (as shown in FIG. 2), and is typically 1.12 eV for silicon (Si) and 1.42 eV for gallium arsenide (GaAs) at room temperature.
Referring to FIG. 3, a common approach at providing a compensating circuit is to utilize an NFET capacitor 14 in an operational amplifier ("op amp") portion 12 (i.e., the circuit components within the dashed line) of a reference voltage circuit 10. While the NFET capacitor 14 provides good voltage stability once steady state operations have been achieved, at power-up the capacitance of the NFET capacitor 14 can vary widely. As a result, the voltage of the reference circuit, which the NFET capacitor 14 is designed to compensate, is often susceptible to large voltage swings, as shown by curve 18 in FIG. 4. In fact, it is common for improperly tuned reference voltage circuits to enter into an oscillating voltage pattern which impedes steady state operation of the circuit (as shown in the area 20 of FIG. 4). Often, these voltage oscillations are induced by the parasitic capacitance present in other components used in the reference voltage circuit (including, for example, resistors 16 (as shown in FIG. 3)).
A parasitic capacitance may arise in a resistor 16 in an integrated circuit because the resistor 16 is commonly formed from a diffusion, which has an associated parasitic capacitance. Capacitors and other electronic components are commonly formed from FETs using known processes, because using a FET is significantly less expensive. A FET component reduces the number of masks needed to produce an integrated circuit.
Ideally, resistors 16 are formed from a diffusion. When an inverse biased junction is applied across the resistor 16, a parasitic capacitance often arises. As a result, the parasitic capacitance of the resistors 16 (and other components) may induce a phase shift in the feedback loop of the reference circuit and may affect the stability of the reference voltage circuitry. Additionally, as shown in FIG. 4, the parasitic capacitance may result in a reference voltage which oscillates and never reaches a steady state value. Additionally, in the reference voltage circuitry 10, the NFET capacitor 14 is a low threshold voltage NFET. A low threshold voltage FET is utilized in the present application because the value of the capacitance for this particular FET starts to increase (i.e., ramp-up) when the voltage between nodes A and B is much lower. Since a low threshold voltage FET can only be built into the substrate of the FET, the substrate of the capacitor must be tied to ground. Thus, an NFET capacitor 14 must be utilized as the compensating capacitor.
Even when the resistors 16 are ideal, the use of FETs as capacitors often results in a capacitor with a varying capacitance. As shown in FIG. 5, when an NFET capacitor 14 is initially powered-up, the capacitance may vary widely depending upon the voltage across the poles (for example, A and B of the NFET capacitor 14 shown in FIG. 3). Thus, as the voltage increases during power-up, the capacitance may vary significantly such that obtaining a steady state capacitance is delayed and often problematic.
As is commonly known, electronic circuits are specifically designed with components operating within a predetermined range of values (capacitance, resistance, etc.). When the component's operating value (in this case the capacitance) fluctuates, the electronic circuit commonly will not perform as expected or as intended. Circuit designers require reliable components that operate within predetermined specifications. Hence, integrated circuit components which do not provide predictable and stable values, such as the NFET capacitor 14 in FIG. 3, must be improved.
Thus, currently available electronic devices do not provide a reliable compensating capacitor modeled out of a low threshold voltage FET.